The output current of a power switching transistor is typically limited, via control circuitry, below a specified level. A representative use for semiconductor switching circuits is within DC-to-DC converter switching power supplies, which utilize a power switching transistor to produce the requisite DC current or voltage output The function of a DC-to-DC converter is to derive power from one source of DC voltage and deliver that power to a load at a regulated DC voltage (the regulated DC voltage may be the same as the originating source voltage or may be different than the originating source voltage, depending upon the design implementation required for a specific application). Various DC-to-DC converter circuit designs incorporating a power switching transistor are known in the prior art, including among others, those utilizing a pulse width modulator to control the conductive state of the power switching transistor. Additionally, various circuit design topologies are also known in the prior art, including, among others, forward, buck, boost, and flyback topology converters. Each individual topology and design utilizes some means of limiting the output current of the power switching transistor.
A conventional DC-to-DC converter circuit design incorporating a power switching transistor with output current limiting control is shown in FIG. 1. The circuit as illustrated in FIG. 1 is arranged with flyback topology. The positive polarity of a DC input voltage, V.sub.in1, is applied to one end of a primary winding of transformer T.sub.11, the second end of the primary winding is coupled to the drain of field effect transistor (FET), Q.sub.11, the source of Q.sub.11 is coupled to one end of source resistor R.sub.12, and the second end of R.sub.12 is coupled to the negative polarity of V.sub.in1, which is grounded. A pulse width modulator (PWM.sub.1) 12 is provided to control whether Q.sub.11 is on (conducting) or off (non-conducting). The PWM.sub.1 control output 14 is coupled to the gate of Q.sub.11 to provide the control function. A primary current sensing input (PCSI) 16 of the PWM.sub.1 12 is coupled to monitor voltage developed across source resistor R.sub.12. A secondary current sensing input (SCSI) 18 is also available as an input to the PWM.sub.1 12.
The secondary winding 20 of transformer T.sub.11 is coupled to provide regulated DC current through rectifyig diode D.sub.11. The cathode of D.sub.11 is coupled to one end of a secondary current sensing resistor, R.sub.11, with the second end of R.sub.11, coupled to a load resistance (R.sub.L1). R.sub.L1 is grounded to provide circuit continuity back to the secondary winding of T.sub.11. Optical isolator (U.sub.11) is comprised of a light-emitting diode D.sub.U and a transistor T.sub.U. D.sub.U is coupled across (and parallel to) R.sub.11. The collector of T.sub.U is coupled to provide a secondary cur-rent sensing signal to the SCSI 18 of the PWM.sub.1 12. The emitter of T.sub.U is grounded.
The DC-to-DC flyback converter of FIG. 1 operates as follows. When PWM.sub.1 12 provides a pulse width modulated control signal to the gate of Q.sub.11, Q.sub.11 turns on (begins conducting). The current path is from the positive polarity of V.sub.in1, through the transformer primary 10 of T.sub.11, through conducting transistor Q.sub.11 (from drain to source), through source resistor R.sub.12, and back to the negative polarity of V.sub.in1. The voltage developed across R.sub.12 is provided as the primary current sensing input (PCSI) 16 for PWM.sub.1 12. When PCSI 16 voltage reaches a predetermined level, PWM.sub.1 12 ceases to provide a pulse width modulated control signal to the gate of Q.sub.11 (i.e., PWM.sub.1 control signal returns to zero) and Q.sub.11 ceases to conduct (shuts off).
Transformer T.sub.11 is provided for isolation protection between the dissimilar voltages of a primary circuit (incorporating the primary winding 10 of T.sub.11) and a secondary circuit (incorporating the secondary winding 20 of T.sub.11). The inductance associated with the primary winding 10 of T.sub.11 causes the creation or enlargement of a magnetic field surrounding the primary winding whenever Q.sub.11 is on. Conversely, the magnetic field surrounding the primary winding collapses whenever Q.sub.11 is off. Due to mutual inductance between a transformer's primary and secondary windings, whenever the magnetic field of the primary winding 10 of T.sub.11, is in a state of flux (that is, the associated magnetic field is enlarging or collapsing), a potential is induced within the secondary winding 20 of T.sub.11. Increasing the frequency of transition between the conducting and non-conducting states of Q.sub.11 causes the coupling between the primary winding 10 and secondary winding 20 of T.sub.11 to increase, ultimately coupling more power to the secondary load circuit through the secondary winding 20.
Rectifying diode D.sub.11 is provided to restrict current induced in the secondary winding 20 of T.sub.11 unidirectionally through load resistance R.sub.L1. The value of filter capacitor C.sub.11 is selected to reduce power supply ripple to an acceptable and tolerable quantity. Current induced in the secondary winding 20 is directed through secondary current sensing resistor, R.sub.11, before passing through load resistance R.sub.L1.
The resistance value of R.sub.12 and the predetermined PCSI 16 voltage which causes Q.sub.11 to shut off are selected to limit current through transistor Q.sub.11 and the associated primary current loop. However, R.sub.12 and PCSI do not constrain the secondary circuit output power available at load resistance R.sub.L1. Specifically, if the unregulated value of inlet voltage V.sub.in1 is allowed to double, output power at R.sub.L1 would quadruple (if no secondary circuit output power control circuitry were provided). Separate secondary circuit output power control circuitry is therefore implemented.
Secondary circuit current control is accomplished by providing a feedback signal corresponding to secondary circuit current to the PWM.sub.1 at the secondary circuit sensing input (SCSI) 18 of the PWM.sub.1. When isolation between dissimilar voltages of the primary circuit (incorporating the primary winding 10 of T.sub.11) and the secondary circuit (incorporating the secondary winding 20 of T.sub.11) is required, one method of providing isolation is through the use of an optical isolator U.sub.11. An optical isolator is a semiconductor device comprised of an light-emitting diode D.sub.U and a transistor T.sub.U that conducts when it receives a photon emitted by the light-emitting diode. The light-emitting diode generates photons when the voltage applied across it is sufficient to forward bias the diode. Since D.sub.U is coupled across secondary current sensing resistor R.sub.11, when current through R.sub.11 develops a voltage sufficient to forward bias D.sub.U, T.sub.U will begin conducting. Once T.sub.U is conducting (on), a secondary circuit current control signal is applied to the SCSI 18 of the PWM.sub.1 12. When the secondary circuit current control signal applied to SCSI 18 reaches a predetermined level, the PWM.sub.1 12 ceases to provide a pulse width modulated control signal to the gate of Q.sub.11 (i.e., the PWM.sub.1 control signal returns to zero) and Q.sub.11 ceases to conduct (shuts off), thereby regulating the current/voltage available at R.sub.L1. Therefore, despite fluctuations in the value of V.sub.in1, outlet voltage at R.sub.L1 is regulated through the combination of primary current sensing and secondary current sensing.
However, several disadvantages exist with conventionally designed switching circuits. First, circuit efficiency is degraded by incorporating secondary current sensing resistor R.sub.11. This is so because all current available to the load resistance R.sub.L1 must first pass through resistor R.sub.L1, thus reducing circuit efficiency. Also, since R.sub.11 is connected in series with R.sub.L1, R.sub.11 characteristically possesses significant power dissipation capabilities and is therefore a large resistor occupying additional circuit board space.
Additionally, in circuits requiring isolation between primary and secondary circuits, the use of an optical isolator (to provide feedback from the secondary winding circuit to the primary winding circuit) also requires additional circuit board space. Furthermore, utilizing a secondary current sensing resistor (R.sub.11) and/or an optical isolator (U.sub.11) requires additional production costs to manufacture the circuit.